1. Field of the invention
The present invention relates to the field of differential polarization interferometry and can be used for the direct measurement of phase shift of a light wave passed under arbitrary angle through the sample consisting of a transparent substrate and deposited on it thin film under test that is rather important at metamaterial testing.
2. Description of the Related Art
The known system of measuring the phase shift in the material, based on the reflective Mach-Zehnder interferometer with two parabolic mirrors in which the incident light is split using non-polarizing beam splitter cube into two beams, object and reference ones. Then these two beams are overlapped using plane mirrors for interference onto the input slit of the spectrometer [1]. The sample was made of a substrate, on the part of which a film under test has been deposited, and it can be shifted in a transverse direction. The interferogram of the sample area containing only a substrate and the interferogram of the substrate with thin film under test are processed with the technique known as spectrally and spatially resolved interferometry (SSRI). The disadvantages of this system are the problems of measuring the phase shift of the sample when it is tilted, as disrupted Mach-Zehnder interferometer due to shear the object beam, as well as lack of sensitivity due to single passing of object beam through a film.
One type of interferometer representative of the state-of-the-art is the differential plane mirror interferometer described in [2]. It comprises a laser source emitting a beam with two orthogonal polarization components, birefringent optical element converting the input beam into two separated, parallel, orthogonally polarized beams. The birefringent element together with two quarter-wave plates, one of which contains two holes, retro-reflector with a pair of holes and two plane mirrors, the first of which contains two holes, causes each of the separated, parallel, orthogonally polarized beams to be reflected twice by one of the two mirrors. The birefringent element converts the two separated, parallel, orthogonally polarized output beams into a single output beam wherein the phase shift between the two polarization components is directly proportional to the optical path between the two plane mirrors. Polarizer mixes the orthogonal components of the single output beam. A photoelectric detector produces an electrical measurement signal. An electronic device is used for signal analysis and extracting the phase shift. This interferometer possesses a number of disadvantages, namely, a complicated optical scheme, requiring adjustment of a many optical components, which contribute to the output beam of reflection from faces limiting the accuracy; operation in a narrow spectral band defined by quarter-wave plates; necessity to measure the thickness of the sample substrate.
An interferometer (walk-off interferometer) [3] is the closest to the technical nature of the proposed technical solution (prototype device). The device comprises the tunable laser source, the first polarizer with a transmission axis at 45° to produce a beam having two orthogonally polarized components; the first birefringent element, which converts these components into two separate, plane, orthogonally polarized beams, one of which is an object one, the other is a reference one; the sample made as a plane-parallel transparent substrate, wherein a part of the surface is deposited with a thin film of the test material, and which can be moved laterally so that object beam passed through the film; half-wave plate swapping polarization of the beams; the second birefringent element that converts orthogonally polarized beams into a single output beam with two orthogonally polarized components; controllable liquid crystal (LC) phase retarder; the second polarizer, parallel to the first one, at the output of which two polarization components interfere; photodetector transforming the result of interference into electric signal, which can be analyzed by various electronic devices.
This interferometer possesses a number of disadvantages: differences in properties of birefringent elements can lead to the non-compensated phase shift between reference and object beams that lowers the accuracy of measurements; initial voltage on the LC phase retarder can get into the area, where large phase shift corresponds to the small one of the controlling voltage that makes worse the accuracy of measurements; operation in a narrow spectral region through the half-wave plate; inefficient sensitivity determined by a single passing of a film by light; rotation of a sample creates problems due to the shift of the two beams, incident on the second birefringent element.